A Static Random Access Memory (SRAM) cell (i.e., circuit) does not need to be periodically refreshed since it uses bi-stable latching circuitry to store each bit. Although a characteristic of an SRAM cell is data remembrance, data stored by the cell is volatile since the data is eventually lost when the cell is not powered.
Each bit in an SRAM cell is a circuit comprising four transistors that form two cross-coupled inverters. This storage cell has two stable states which are used to denote zero and one. Two additional access transistors serve to control the access to a storage cell during read and write operations. Access to the cell is enabled by a word line (WL) which controls the two access transistors which, in turn, control whether the cell should be coupled to bit lines BL and BL (i.e., each bit line coupled to a respective inverter). The bit lines are used to transfer data for both read and write operations.
During read operations, the bit lines are actively driven high and low by the inverters in the SRAM cell. The read operation starts by pre-charging both the bit lines to a logical one, then asserting the word line, enabling both access transistors. The values stored in the inverters are then transferred to the bit lines by leaving BL at its pre-charged value and discharging BL to a logical value of zero. On the BL side, the bit line is pulled toward Vdd (i.e., a logical value of one). If the content of the memory were a “0”, the opposite would happen and BL would be pulled toward “1” and BL toward “0”.
The start of a write cycle begins by applying the value to be written to the bit lines. If we wish to write a “0”, we would apply a logical zero to the bit lines by setting BL to a logical one and BL to a logical zero (a one is written by inverting the values of the bit lines). The WL is then asserted and the value that is to be stored is latched in. In order to implement a write operation, the bit line input-drivers are designed to be much stronger than the (relatively) weak transistors in the cell itself, so that they can override the previous state of the cross-coupled inverters.